A thermoelectric generator (TEG) is a device that can generate electricity when a temperature differential is applied across the device. One example of a TEG device is a thermal semiconductor chip, such as those manufactured by ENECO, Inc. of Salt Lake City, Utah. A TEG device is typically square or rectangular with the upper and lower end-caps having the same dimension and typically power generated by TEGs is transmitted via a set of power wires. TEG devices are typically thin (e.g., in the order of a couple of millimeters thick), small (e.g., a couple of square centimeters), flat, and brittle. Accordingly, TEG devices can be difficult to handle individually, especially for applications in vehicles, such as automobiles, aircraft and the like, where the TEG devices can be subject to harsh environmental conditions, such as vibration, constant temperature variations and other harsh conditions. Because of their size and the fact that each TEG device generates only a small amount of power, many TEG devices are bundled together in order to generate a useful amount of power. Further, TEG devices generally provide greater energy conversion efficiency at high temperature. This can cause relatively large thermal expansion in materials. Because of thermal gradients and different thermal coefficients of expansion associated with different materials, thermally induced stresses may result.
Efficiency of TEG devices generally increases with greater temperature differentials, i.e., delta temperature between two opposite sides, typically called the heat source (hot side) and heat sink (cold side) of the TEG device. Also, energy conversion efficiency is maximized for any installation that channels heat flow through the TEG devices only without any thermal energy leaks through the surrounding structural material or gaps. Thus, to simplify handling and achieve high performance in converting heat to electricity, multiple TEG devices can be encased into a module or assembly prior to final installation. FIG. 1 is an illustration of an exemplary prior art thermoelectric generator assembly 100 or module. TEG devices 102 are sandwiched between two structural plates 104 and 106. Each of the two structural plates 104 and 106 may be made of a thermally conductive material, such as copper or Aluminum Nitride (AlN), to spread the heat on both hot and cold sides of the TEG module 100. One of the plates, such as upper structural plate 104 may define a cold spreader plate and may be thermally coupled to a cold side 108 of each of the TEG devices 102. The other plate, such as the lower structural plate 106 may define a hot spreader plate and be coupled to a hot side 110 of each of the TEG devices 102. Each of the plates 104 and 106 may be respectively thermally coupled to the cold side 108 and hot side 110 of each of the TEG devices 102 by a brazed interface 112 and 114. Vacuum gaps 116 or insulation material may be used to separate each TEG device 102 in the module 100 to maximize heat flow through TEGs 102. Additional insulation may be required to prevent heat losses through the sides.
FIG. 2 is an illustration of a cross-sectional view of a prior art thermoelectric generator (TEG) module 200 for installation in an aircraft engine compartment or similar apparatus for generation of electrical energy. The TEG module 200 may also be referred to as an Extended Mission through Energy Conversion (EMTEC) module. Heat energy 202 may be received from a heat source 204, such as an interior of a gas turbine engine of an airplane. The heat energy 202 is received by a thermal interface or heat spreading plate 206 similar to that previously described which may be made of a material such as copper, aluminum nitride (AlN) or similar heat conductive material. The thermal interface or heat spreading plate 206 may be thermally connected to the TEG device 208 by brazing or by some other means to provide a negligible resistance to the transfer of thermal energy to the TEG 208. In this installation approach, portions of insulation material 210 and 212 may be used to fill the gaps between adjacent EMTEC modules to prevent heat escaping through the space between the modules. Another portion of insulation material 214 may also be disposed on each edge of the TEG 208 for efficient transfer of heat energy through the TEG 208 for generation of electricity. A cold side interface 216 or cold spreader plate similar to that previously described may be thermally coupled to the cold side of the TEG 208. Both the EMTEC module 200 and insulation material are attached to a stress buffer layer 218 which is attached to a cold sink 220 or heat sink. A stress buffer layer may also be used on the hot source as well. The stress buffer layer 218 acts as an absorption layer for thermal expansion mismatch induced stress and vibration. Such a “bridge” material between two materials that are incompatible in regards to coefficient of thermal expansion (CTE) is typically selected to have a CTE in between those of the two incompatible materials. Installation of the EMTEC module 200 to the cold sink 220, for example, the exterior wall of an aircraft engine compartment can be accomplished by various means, e.g., mechanical means by attaching insulation and the stress buffer layers 218 to the cold sink 220 or the engine compartment wall with bolts, etc. and bonding means by attaching EMTEC modules 200 to the stress buffer 218 using thermal grease, soldering, or brazing for high temperature application.
Each EMTEC module 200 needs to be individually attached or adhered to the insulation material or stress buffer layer 218. This is a labor intensive operation and adds cost to manufacturing. Additionally, brazing TEGs is a delicate process and damage during manufacturing can often occur. Because more power is generated when TEGs are installed in locations with the greatest temperature differentials, TEG devices are potentially subjected to high temperature. To avoid thermal stresses generated by uneven thermal expansion between TEGs and the structural material, the structural material is chosen based on its thermal expansion quality over its thermal conductivity. Less than desirable thermally conductive material leads to reduced power generation. For units that are installed in a system where periodic variations in temperature are common, such as an engine, thermal expansion for each material becomes a significant design challenge.
Each layer of material in the thermal path from the hot side to the cold side of a TEG module, such as module 200, adds thermal resistance to the overall thermal stack. Increased thermal resistance reduces effective temperature differential across the TEG, and loss of energy conversion efficiency can result. More layers in the thermal path of the TEGs results in higher thermal resistance, which in turn reduces the overall system efficiency.
Another drawback for some applications is that EMTEC modules are flat. The surface of the high temperature source may not be flat. For example, the surface of an aircraft turbo engine is cylindrical and the diameter of the cylinder varies along its length.